Computer-aided simulation is a powerful aid during the design or analysis of electronic circuits and semiconductor devices. This chapter focuses on analog circuit simulation and covers simulations of semiconductor processing and devices. While the main emphasis is on analog circuits, the same simulation techniques may, of course, be applied to digital circuits (which are, after all, composed of analog circuits). The main limitation will be the size of these circuits because the techniques presented here provide a very detailed analysis of the circuit in question and, therefore, would be too costly in terms of computer resources to analyze a large digital system. The most widely known and used circuit simulation program is simulation program with integrated circuit emphasis (SPICE). The chapter considers the three primary simulation modes: DC, AC, and transient analysis. A number of programs are available (either from universities or commercial vendors) which can model silicon processing. The best known program is SUPREM-IV.